Machine vision is the analysis of images by a computing device such that some determination can be made about a scene or object that is viewed by an image sensor or sensors connected directly or indirectly to the computing device.
Electronic integrated circuits (ICs) are generally enclosed in plastic or ceramic packages of a generally rectangular parallelepipedal hexahedral shape. These packages include a plurality of leads about the periphery to provide electrical contact to the circuits within the package, and the leads include a means for attachment to a printed wire circuit board that typically forms part of a larger assembly of similar integrated circuits and other electronic components.
The old standard practice generally deployed the leads to protrude from the top and/or bottom major surfaces of the package such that all the leads were oriented in a direction normal to the plane of the major surfaces of the package. These leads were then inserted through corresponding holes in the printed wire board for mechanical and electrical attachment by soldering. These packages are known in the art as through-hole packages.
In the last 10 to 15 years, conventional practice has transitioned to deploying the leads about the periphery of the IC package in a plane parallel to the package with the distal portions of the leads offset from the plane of the package. The distal portions of the leads are then positioned on corresponding pads on the printed wire boards for soldering. This technique allows closer spacing of the leads, a reduction in size of the overall package, and the placement of IC packages on both sides of the printed wire board. These packages are known in the art as surface-mount-technology (SMT) packages.
FIG. 1 is an isometric view showing a conventional SMT package 10 having a defective lead 12 among a plurality of leads 14a-14j (collectively or generically, 14) that protrude from major side surfaces 16 and/or minor side surfaces 17 of a plastic or ceramic casing 18. With reference to FIG. 1, for each lead 14 of SMT package 10 to be successfully soldered to a substantially planar printed wire board (not shown), leads 14 should all be substantially within the same plane, a property known in the art as lead coplanarity. For example, a defective or noncoplanar lead 12 may exhibit a property known in the art as lead standoff, a defect characterized by a lead 12 that sits too high above a printed wire board for a reliable solder connection to be made between the lead 12 and a corresponding copper pad on the printed wire board.
The methods of determining the coplanarity of leads 14 of SMT packages 10 can be divided for the purposes of discussion into two groups: those that can generate a three-dimensional surface map of an object with one view and those that synthesize a three-dimensional map of the object viewed by combining the information from a plurality of two-dimensional maps.
The first group includes techniques that use a particular type of illumination with a property that renders a two-dimensional image with additional information on the distance of each point in the image from the image sensor. Examples of such techniques include structured lighting techniques, scan-angle modulated techniques, moire techniques, interferometric techniques, holographic techniques, and time-of-flight techniques. For example, Beiman and Michniewicz disclose a moire interferometry technique for measuring surface contours of an object in U.S. Pat. No. 5,636,025.
The second group uses a plurality of two-dimensional images that are generated by a plurality of two-dimensional image sensors such as conventional video cameras and uses the a priori knowledge of the positions of the image sensors to mathematically construct the three-dimensional nature of the object viewed.
As a convenience, to reduce the size of the coplanarity sensing device, and as an economic measure to reduce the number of image sensors required, two or more of the required views may be combined by mirrors or by prisms, or by a combination of both, onto a single two-dimensional image sensor.
For example, Lebeau and Hopkins disclose a technique employing at least one mirror and a pedestal 20 that serves as a physical reference plane 22 for determining SMT lead coplanarity in U.S. Pat. No. 5,563,703 ('703 patent). Smeyers and Vanderheydt similarly disclose a technique employing a plurality of mirrors, a pedestal 20 that serves as a physical reference plane 22, and a diffusive top surface 24 upon which a shadow of leads 14 are cast from a plurality of angles to determine SMT lead coplanarity in U.S. Pat. No. 5,440,391 ('391 patent). In FIG. 1 and with reference to the '703 and '391 patents, the top surface 24 of pedestal 20 provides the reference plane 22, which is also commonly known as the Z-axis reference plane 22. The height, or Z coordinate, for each lead 14 is then determined with respect to the physical reference plane 22.
These inspection systems require specialized handling devices, such as a Z-axis actuator, capable of deploying an SMT package 10 along a specified axis or path to position SMT package 10 to be substantially in contact with and coplanar with the surface 24 of pedestal 20 at a predetermined fixed location that is within a view of an image sensor. The view is from above, but not normal to, the physical reference plane 22.
The typical requirement for motion along the physical reference plane places two burdens on SMT package inspection. One burden is the requirement for a Z-axis actuator and its attendant power and control devices. The other burden is the time required to deploy the SMT package 10 in the Z axis onto pedestal 20, the pause required during which the images of SMT package 10 on pedestal 20 are acquired, and the time to reverse the deployment of SMT package 10 from pedestal 20 so that SMT package 10 may resume travel along its original path.